In the efforts for optimizing and improving operation in various high-speed microcontroller-based devices, such as various instrumentation and measurement equipment and the like, significant attention has been given to the further improvement of the high-speed operational amplifiers utilized. For example, efforts have been made to improve transimpedance amplifiers that are used to convert low-level photodiode currents to usable voltage signals, and are commonly implemented within low-current and leakage current measurement applications, as well as other low-level sensor current applications.
Many operational amplifiers are being designed for improving not only the common mode rejection ratio (CMRR), noise and offset, but also the rail-to-rail performance. For example, operational amplifiers are desired with an output stage having full rail-to-rail output swing capability, such as in many transimpedance amplifier applications, e.g., from below zero volts or ground to above a full-scale voltage of VDD. With reference to FIG. 1, an output stage 100 as may be implemented within a conventional operational amplifier having a class AB output configuration is illustrated. Output stage 100 comprises a pair of output transistors, M1 and M2, operating in a common-source configuration. Output transistors, M1 and M2 are typically configured as PMOS and NMOS devices, with PMOS output transistor M1 having an input terminal, e.g., a source terminal, coupled to a supply voltage, and with NMOS output transistor M2 having an input terminal, e.g., a source terminal, coupled to ground.
For most single supply operational amplifiers, difficulties arise when the output signal is pulled downwards to the lower output swing limit, such as to ground or below. Unfortunately, even the better amplifiers and output stages are only able to swing close to single supply ground, for example within approximately 10 mV, well short of the approximate 1 μV or less that is desired. Further, when output voltage at VOUT is close to zero volts or ground, output transistor M2 will tend to operate within the deep triode region, acting effectively as a resistor, thus resulting in low gain complications, and thus slower speeds.
One method to improve the pulling downward of the output to ground or zero volts includes the addition of a pull-down resistor RPD coupled between output voltage VOUT and a negative voltage, e.g., ground less a voltage VPD. Pull-down resistor RPD is configured to facilitate pulling down of output voltage VOUT towards or below the lower supply. However, pull-down resistor RPD does not address the lower gain characteristics caused by output transistor M2 during pulling-down of output voltage VOUT, output stage 100 still realizes reduced speed.
Other attempts to effectively pull output voltage VOUT downwards to or below the lower output supply, many operational amplifiers are configured with dual supply voltages instead of single supply voltages. However, such dual supply operational amplifiers generally require higher voltage processes than the low voltage processes utilized by single supply operational amplifiers.